It is not a spot welded pice. It is more of continuos seam weld. It looks like a toothed wheel was used to press and weld it together so there is no leaks. It would be impossible to separate it. It was much easier to do what I did. I did the cut and weld on the top so you cannot see it once the tank is on the car.

whee! doing it the way I did it would be simple to hammer out the folds and make it look good.

Glad you chipped in 240dkw. That was the thread I was refering to, but I couldn't find the thread again. I was impressed with the work you did to it.

I will do some more research on the tank construction. There is no weld that I know of around the perimeter of the tank flanges. I can't think of a fabrication technique that Nissan would have used 40 years ago that would have left those kind of marks... anyone have any ideas?

In this video

 

https://www.youtube.com/watch?v=i3D7PuyXXp0

 

What is the machine at 3:31 and how does it work? Is it welding with the 2 wheels, friction welding or crimping or what, looks like there is power to the heads and some kind of vent drawing off smoke...

In this video

 

https://www.youtube.com/watch?v=i3D7PuyXXp0

 

What is the machine at 3:31 and how does it work? Is it welding with the 2 wheels, friction welding or crimping or what, looks like there is power to the heads and some kind of vent drawing off smoke...

It's a seam welder. In a way like a spot welder but for a continuous weld. A lot of catalytic converter are seam welded. High voltage with the copper disc providing pressure.

Yes it is called resistance seam welding  I think they used a toothed wheel to make the crimps as it welded

Edited December 4, 20159 yr by 240dkw

240dkw

When you cut your tank, it looks like you cut up high enough to cut into the air of the tank with a blade? Is there enough room on the flat to take a die grinder and grind through the shell following a line just inside the resistance weld? That way when you reassemble the tank you can basically weld it backed up by the other half of the shell. Butt welding all around the tank seems like the harder option. My concern for me would be making 8' or so of weld with no pin holes or blowing bigger holes

I could run a sawzall around or a cutting wheel... less spark with a sawzall, less deformation with a cutting wheel.....

If I were gonna try it I would get the tank cleaned real well and take a cutoff wheel around just inside the resistance weld seam. That should give you a good way to weld it back together.

My tank sat on a shelf for the better part of two years before I cut it apart so I did not worry about gas vapours. I used a cutting wheel as close to the seam as I could get without cutting through the bottom part. The only problem with a sawzall is that the blade will catch on the baffle plate and maybe on of the tubes, that is why I went with the wheel. The tank is made of a heavier gage steel so blowing through was not  problem like it is on the body steel. I did use a respirator rated for lead fumes when I did the welding with both doors open in the garage and a good cross ventalation. as these tanks had a lot of leaded gas in them I did not want to take in any fumes. 

Is there anyway to use air pressure to pop the dents out?  I used to fill the metal tank on my dirtbike with water and freeze it.  takes a few times but it would get small dents out of the tank.  Freezing a car tank would be hard to do though, too many vent holes and you'd have to have a deep freezer or a restaurant's walk in.  You could spot weld a hook on it and pull it out like auto body shops do.  Maybe get a dent puller on a loaner program from local auto parts?  Just ideas.